Controlling corrosion kinetics of magnesium alloys by electrochemical anodization and investigation of film mechanical properties

Applied Surface Science 484 (2019) 906–916

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Controlling corrosion kinetics of magnesium alloys by electrochemical anodization and investigation of film mechanical properties

T

Zia Ur Rahmana,c, K.M. Deenb, Waseem Haidera,c,

⁎

a

Science of Advanced Materials, Central Michigan University, Mt. Pleasant, MI 48859, USA Department of Materials Engineering, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada c School of Engineering and Technology, Central Michigan University, Mt. Pleasant, MI 48859, USA b

ABSTRACT

The rapid corrosion of magnesium alloys in physiological environment constraints its applications to employ it as a biodegradable implant material. AZ31 and ZK60 alloys were executed to anodization in alkaline solution as a function of time and investigated their electrochemical and mechanical properties. The scanning electron microscopy reveals the compact passive film formation subsequently to anodization. The high-resolution spectra of x-ray photoelectron spectroscopy confirmed the existence of MgO, Mg(OH)2 and traces of CO3−2 within the anodized layer. The quasi-static with displacement-controlled mode of indentation was performed to investigate the mechanical properties of the anodized films. Anodized ZK60 has an average hardness of ~0.49 GPa, greater than the hardness of anodized AZ31-Ano (0.35 GPa). Similarly, the stiffness and film elastic modulus for anodized ZK60 were higher when compared with anodized AZ31. The surface roughness of anodized ZK60 (473.54 ± 51.61 nm) was higher than that of anodized AZ31 (112.11 ± 11.31 nm). The potentiodynamic polarization scans for AZ31 showed corrosion current density of 4.46 μA/cm2 (untreated) and 394.8 e−3 μA/cm2 (anodized), while in ZK60 corrosion current densities values shifted from 12.05 μA/cm2 (untreated) to 714.8 e−3 μA/cm2 (anodized). Similarly, electrochemical impedance spectroscopy indicated enhanced charge transfer resistance for anodized AZ31 (1.164 KΩ-cm2) and ZK60 (1.911 KΩ-cm2) when compared with untreated AZ31 (28.3 KΩ-cm2) and ZK60 (0.481 KΩ-cm2). Furthermore, MC3T3 preosteoblast cells on the anodized surfaces caused no adverse effects in regards to biocompatibility.

1. Introduction The tendency towards the bioresorbable implantable material is getting attention due to the increase in a number of orthopedic and cardiovascular traumas. In this regards, magnesium and its alloys have been attracting the consideration of the researchers, as a permissible implant material to support the structure temporarily, and safely degrade in a body. The advantage of the bioresorbable materials would reduce the chances of inflammations usually caused by permanent or non-degradable materials by prolonging infusion in the human body. Although, some of the permanent implants are compatible to some extent, in most cases its surgical extraction is obligatory after completing its tasks, which is excruciating and costly at the same time. Magnesium is a promising biodegradable implant material for cardiovascular and orthopedic implants because of its non-toxic nature [1–3]. Its high strength to weight ratio makes it is an ideal metal for bone implant applications [4]. Due to its mechanical properties, magnesium and its alloys reduce stress shielding in bones and stand in as new generation implantable material [5,6]. These alloys considered as ideal to use in a human body as a temporary implant material. Their unique property of dissolution in physiological environments eliminates the need for revision surgery [7]. This is an important advantage

⁎

compared to the conventional biomaterial. The generally use metallic implants consist of stainless steel, cobaltchrome based alloys, titanium, zirconium, and tantalum [8,9] [10,11]. Former applications emphasize the biomaterial to stay in the body forever, which generally leads to the adverse reactions generating the localized inflammation and metallosis [12]. The long-term complications in implant necessitate the complete removal of the implant and thus require revision surgery. Furthermore, permanent metallic implants not in complete accord to the tissues and the mechanical mismatch usually result in stress shielding, in which the bone is undergoing in remodeling due to the unusual stresses applied in daily activities and results in bone resorption [13,14]. However, magnesium alloys degrade non-uniformly and high corrosion rate in the physiological environment make it unsuitable due to loss of mechanical integrity and leads to the collapse of implants. The physiological body fluids with a strong concentration of ions can aggressively attack the magnesium produces hydroxide ions and hydrogen gas, results in the significant increase in local pH, which could lead to implanting structural failure as well as other biologically adverse outcomes. Different methods have been employed to overcome the degradation issue of the magnesium and its alloy [15–17]. The previous studies

Corresponding author at: Science of Advanced Materials, Central Michigan University, Mt. Pleasant, MI, 48859, USA. E-mail address: [email protected] (W. Haider).

https://doi.org/10.1016/j.apsusc.2019.02.168 Received 19 October 2018; Received in revised form 31 January 2019; Accepted 18 February 2019 Available online 19 February 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

Applied Surface Science 484 (2019) 906–916

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show that alloying elements with magnesium are the possible way to bring enhancement in corrosion properties [18,19]. However, some alloying elements of magnesium increase the yield strength, ductility, and ultimate tensile strength while decreases its hardness [20]. While other decreases in elongation and tensile strength [21]. Therefore, considering the above challenges, altering surface texture and properties are thought to be successful to enhance control degradation [22]. Anodization technique is the electrochemical treatment that alters the surface by oxidation, producing a stable metal oxide layer, resulting in the deceleration of the magnesium alloy corrosion rate in the human body [23]. The oxidized layer act as a barrier to the corrosion of magnesium. The structure of the oxide layer is characterized by a dense oxide layer, followed by a porous oxide layer [24]. In this present work, the magnesium alloys were anodized in an alkaline electrolyte by optimizing voltage and time. The surface morphology and topography of the magnesium alloys were studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM) while surface chemistry was studied using energy dispersive spectroscopy (EDS) and x-ray photoelectron spectroscopy (XPS). The mechanical properties of anodized alloys were investigated by using nanoindenter. Furthermore, potentiodynamic polarization scans and electrochemical impedance spectroscopy curves were obtained to estimate the corrosion tendency of magnesium alloys. The morphology and proliferation of MC3T3-E1 pre-osteoblast cells were observed to evaluate the biological response of the modified surface.

analysis coupled with the SEM (Hitachi 3400). Furthermore, to investigate the composition of the anodized surface films, X-ray Photoelectron Spectroscopy (XPS) was conducted using Kratos Axis Ultra X-Ray Photoelectron Spectrometer. The system was composed of monochromatic Al X-ray source having an energy resolution of 0.5 eV and a take-off angle of 45°. To deconvolute the high-resolution spectra Gaussian and the Lorentzian fitting procedure was adopted to evaluate the chemical state of the species present on the anodized film. 2.4. Electrochemical investigation Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) analysis of the untreated and anodized alloys was carried out to assess their electrochemical performance. The phosphate buffer saline (PBS) was used as an electrolyte, prepared by dissolving one tablet (Part # P4417-50TAB Sigma Aldrich® USA) in 200 ml of deionized water. Gamry potentiostat reference 3000 was used for the electrochemical test. The electrochemical cell was a three-electrode cell setup, in which a saturated calomel electrode (SCE) was reference and graphite was a counter electrode. The magnesium alloys samples were used as the working electrode. For potentiodynamic, tests were within −2.0 to 0.5 V vs. SCE potential range and 1 mV/s scan rate was selected. The exposed surface area of the working samples was (2.02 cm2) in each test. During EIS, the potential perturbation of 10 mV (rms) at 0 V vs. OCP was applied within 0.01 Hz–100 k Hz frequency range. All electrochemical tests were done at 37 °C.

2. Experimental

2.5. Quasi-static controlled displacement nanoindentation

2.1. Materials

Quasi-static with displacement-controlled mode was carried out by using Hysitron Tribo-indenter (TI-950) for the indentation tests. The nanoindenter transducer mounted with the Berkovich tip was calibrated before the test. Each hysteresis plot of load and unload was conducted across the film thickness. Each test was accomplished in triplicate for reproducibility. Tip of the indenter was driven at a constant rate of 0.05 nm/s. In order to avoid the Mullins effect, each indentation was performed at a new location on the sample [25]. The Oliver and Phar method [26] was applied to the hysteresis plots to calculate the stiffness (S), reduced elastic modulus (Er) and hardness (H) values.

The AZ31B (3.0 wt% Al, 1.0 wt% Zn, Mg balance) and ZK60A (6.0 wt% Zn, 0.45 wt% Zr, Mg balance) rods were cut into discs of dimensions 19 mm diameter and 4 mm thickness. The samples were mechanically grounded by using 120 up to 1200 grit SiC paper. The surfaces of the samples were cleaned and degreased in ethanol ultrasonically. 2.2. Anodization Anodization was performed in an electrolyte having the composition shown in Table 1. The electrolyte was prepared in a beaker and the temperature was kept constant at 60 °C. A DC power supply (Sorensen SGA 100X50C-0AAA) was used for anodization. The positive and negative terminals of the power supply were connected with the working sample and graphite rod, separately. The exposed working to graphite counter electrode surface area ratio was 0.9 whereas the distance between these electrodes was kept approximately 2.5 cm. The 20 V potential was applied and the residence time for anodization was varied for 10, 20 30 and 40 min. During the anodization process, it was observed that the temperature was raised from 60 °C to 65 °C. The procedure was repeated for each sample and each time fresh electrolyte was used. In this study, anodized AZ31 and ZK60 samples are labeled as AZ31-ANO and ZK60-ANO, respectively.

2.6. Cell morphology assessment Biocompatibility of each AZ31-Ano and ZK60-Ano was estimated from the proliferation of MC3T3-E1 pre-osteoblast cells. The cells were cultured in minimum essential medium alpha containing 10% FBS (Fetal Bovine Serum) and 1% Penicillin-Streptomycin. The incubator temperature was kept at 37 °C in a humidified atmosphere under 5% CO2 cover. After 100% confluency of the cells was achieved, the cells were split and are were counted (2 × 104 cells) using countess II FL (Life Sciences) and applied to each surface of the untreated and anodized surface. The samples were incubated for 48 h and the fixation procedure was carried out. The cell fixation was performed using 1.25% glutaraldehyde in 0.1 M PBS (pH = 7.2) and samples were placed in a refrigerator (4 °C) for 24 h. After refrigeration, the samples were rinsed twice with 0.1 M PBS and were dehydrated using 50%, 75%, 80%, 95% and 100% ethanol for 15 min. Followed by the dehydration process, the samples

2.3. Surface characterization The surface of the untreated and anodized specimens was examined in the SEM and chemical composition was determined to form the EDS

Table 1 Chemical composition of electrolyte used for the anodizing of AZ31B and ZK60A alloys. Substance

Ethylene glycol

Methanol

Ammonium dihydrogen phosphate

Deionized water

NH4F

pHa

Percentage

200 ml

50 ml

0.1 M

50 ml

0.1 M

14

a

pH was adjusted by the addition of KOH. 907

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Fig. 1. Surface topography of magnesium alloys before and after electrochemical anodization, (a) AZ31-Unt, (b) ZK60-Unt, (c) AZ31-Ano and (d) ZK60-Ano anodized at 20 Volts for 30 min.

were transferred immediately in the biological hood for 24 h drying. To avoid surface charging the samples were sputtered with gold for SEM analysis.

as shown in the inset of Fig. 1. Increasing oxide concentration specifies that the anodic film formed on the sample is composed of magnesium oxides (MgO) and hydroxides (Mg(OH)2) [29] confirmed by FTIR. The surface layer primarily composed of Mg, O, F, Zn, Al and P in the case of AZ31-Anodized whereas Mg, O, F, Zn and P on the surface of ZK60Anodized. According to the possible chemistry of the electrolyte, MgO, and Mg(OH)2 could be the possible composition of the new coating with the content of F and PO4. The surface chemistry and the composition of the anodized layer FTIR (ATR) were carried out and the results were compared with the untreated surface of the alloys. The IR spectra of the untreated samples of both AZ31 and ZK60 exhibit the fluctuated peaks in the range of 2300–1800 cm−1. The spectrum in this range is attributed to the diamond crystal, used in ATR spectroscopy [30]. For an anodized sample of AZ31 sharp peaks at 3696, 3323, 1645, 1413.8 and 1041.35 cm−1 are observed. Whereas, an anodized sample of ZK60 exhibits peaks at 3696, 3323, 1647, 1412 and 1044 cm−1. In the anodized samplesAZ31Ano and ZK60-Ano, the sharp peaks at 3696 cm-1 in the IR spectra are associated with the OeH band stretch of Mg(OH)2 [31,32], indicated that the anodization forms Mg(OH)2 on the surface. The broadband centered at 3323 cm−1 belongs to stretching vibration of the hydroxyl groups, which came from OH group in water that forms strong H bonding inside the structure. In AZ31 and ZK60 a band is observed around 1645 and 1647 cm−1 respectively. The band in this range arises from the bending vibrations of H2O molecule, usually due to the adhesion of the ambient moisture. The peaks at 1400 range correspond to the presence of the CO3−2. Prominent bands at 1041.35 and 1044 cm−1 can be observed in AZ31 and ZK60 spectra, these bands correspond to phosphate as shown in Fig. 2 [33]. To study the effect of anodization time on the properties of the anodized film, the samples were anodized for different time spans at a constant voltage of 20 V. The electrochemical dissolution tendency of

3. Results and discussion 3.1. Physical characterization The SEM micrographs show the topographies and chemical compositions of the substrates before and after anodization in Fig. 1. The micrographs expressed that the surface of each alloy is altered at the substrate/electrolyte interface, under the action of anodic voltage, which directs the enhancement of the oxide film. Nevertheless, the untreated surfaces have visible grinding marks across the entire surface, with uniformity in a unidirectional pattern while the anodized surfaces became texturized and the grinding marks vanish from the surface after the electrochemical process of creating an oxide film, anticipated to increase the corrosion resistance. The new topographies demonstrate micro patterns all over the anodized surface for ZK60 and AZ31 however; AZ31 morphology displays the cracks on the surface. The previous studies stated the micro-cracks in the anodized layer are attributed to the processes, in which the ingress of the anionic species to the oxide layer occurs and act as an impurity. When a voltage is applied during electrochemical anodization these ions are further ionized which results in the release of avalanches of electrons in the oxide layer and causes a breakdown of the surface film [27,28]. Thus, the breakdown causes micro-cracks, which influences the corrosion properties of the material, discussed in detail in electrochemical part of this study. To evaluate the surface chemistry, the EDS tables clearly indicate the increasing amount of oxides in both cases of anodized i.e. AZ31 and ZK60 substrates, when compared to their respective untreated samples 908

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anodized samples was estimated by potentiodynamic polarization. The corrosion current densities (icorr) and corrosion rates were calculated by Tafel extrapolation method and the values are plotted in Fig. 3a and b for AZ31 and ZK60 anodized samples, respectively. After 10 min of anodization of AZ31, highest corrosion rate was observed which was related to the small thickness of the defective anodized film. However, upon extended anodization time (20 and 30 min), almost linear growth in the film thickness (as shown in Fig. 3c) resulted in the decrease in corrosion rate. Similar trends of decrease in corrosion rate and increase in film thickness was observed for anodized ZK60. However, increase in anodization time i.e. 40 min, the sharp increase in corrosion rate for both alloys was attributed to the adverse reaction of ionic species with the anodized film. In other words, under applied conditions, it is considered that the stability of the oxide film was decreased due to the formation of soluble complexes with the anionic species in the electrolyte. Also, by comparing the EDS results of untreated and anodized samples, it was estimated that presence of some of the alloying elements in these alloys would preferentially dissolve under extended exposure, resulting in the formation of defective film thickness. It is considered that the compromise between field assisted the growth of oxide film and preferential dissolution of alloying additions in these alloys would decide the morphology and barrier characteristics of these anodized film as evident from the film thickness and icorr trends in Fig. 3. On the basis of these results, the anodization at 20 V for 30 min is suggested as an optimized condition for both AZ31 and ZK60 alloys. The optimum thickness of about 5.8 ± 0.5 μm in case of AZ31 and 7 ± 0.5 μm in case of ZK60 was achieved at 20 Volts for 30 min. The surface topography was further analyzed AFM images shown in Fig. 3d. The 30 × 30 um2 area was scanned at tapping mode and it was observed that ZK60-ANO has comparative higher nanoroughness (473.54 ± 51.61 nm) than comparison with AZ31-ANO (112.11 ± 11.13 nm). The surface roughness greatly influences the adhesion of the cells by providing nano-topographical ledges to support the extracellular matrices (ECM) of the cells [34,35].

AZ31-Ano

O gM H O P- 3− 4 PO

C

2−

O3

H

H O

H O

g(

C-

M )2

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm )

Transmittance (a.u.)

ZK60-Unt ZK60-Ano

-H

3500

CO

3 2−

-H

4000

O H) 2

C

M g( O

3000

2500

2000

1500

M PO Pg3 O O 4 − H

1000

500

Wavenumber (cm-1) Fig. 2. ATR spectra of AZ31 and ZK60 before and after anodization.

(a)

1.4x10-5

(b)

12

18 2.0x10-5

Icorr (A/cm2)

8 6

7.0x10-6

4 3.5x10-6

2

16 14

Icorr (A/cm2)

1.0x10-5

Corrosion Rate (mpy)

10

1.5x10-5

12 10

1.0x10-5

8 6

5.0x10-6

4 2

0 0.0

0.0 Untreated

10

20

30

40

0 Untreated

Anodization Time (Minutes)

(c)

Corrosion rate (mpy)

Transmittance (a.u.)

AZ31-Unt

(d)

10

20

30

40

Anodization Time (Minutes)

8.0 AZ31 ZK60

Film Thickness ( m)

7.0 6.0 5.0 4.0 3.0 2.0

10

20

30

40

Anodization Time (Minutes)

Fig. 3. Influence of anodization time on corrosion current densities and Corrosion rates (a) AZ31-ANO (b) ZK60-ANO (c) Film thickness with Anodization time and (d) Crossectional of the coating and AFM images of coating topographies at 20 V 30 m. 909

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3.2. Mechanical behavior of anodized film

difference in coefficient of friction or surface interactions with the indenter [37]. Furthermore, with an increase in the maximum indentation depth the peak loads increases, however, the peak loads are higher in case of ZK60-Ano in comparison with AZ31-Ano, which indicates that higher energy is required for ZK60-Ano to cause permanent deformation in the film. Similarly, the slope of the unloading section of each P-h plot also depicts an increase as the maximum indentation depth increases in both samples, which is an indication of contact stiffness of the samples. Furtherly, higher unloading slopes also yield relatively higher elastic modulus and hardness. In the loading section of the load-displacement plot of ZK60-Ano features of multiple pop-ins are evident. The multiple pop-ins are noticeable over a wide range of indentation load and penetration depth, which specify the plastic deformation of the film [39]. These phenomena are usually attributed to dislocation nucleation and propagation or micro-cracking during loading and have been observed in a wide variety of materials [38,39]. In order to study the depth dependence variations in mechanical behavior of the films, the derived graphs of H, Ef, and S as a function of displacement are depicted in Figs. 4b and 5b. The profiles show that at the lower indentation depths, i.e. 1 & 2 µm, of the film, the hardness values for ZK60-ANO is twice higher (~0.58 & 0.41 GPa) as compared to AZ31-ANO (~0.28 & 0.21 GPa), which could be due to the degree of porosity of the top oxides and hydroxides layers of the anodized film that plastically deforms with increasing applied load. At 3 µm depth, the hardness values of both the samples are comparable, which could be due to the substrate effect. The variations of hardness with depth can be attributed to the porosity and mico-cracks within the film. Similarly, Ef values are slightly higher in ZK60-Ano in comparison with those of AZ31-Ano. In case of AZ31-Ano, the film elastic modulus increases with indentation depth, i.e. Ef increases from 7.8 GPa at 1 µm and reaches ~20 GPa at 3 µm. However, in case of ZK60-Ano, the Ef values remain about ~20.04 GPa for all indentation depths. The high hardness and elastic modulus values of the ZK60-Ano film might be attributed to the lower intrinsic coating defects as well as the atomic structure of the film. From the AFM images in Fig. 2, it has been revealed that the granular structures are smaller in case of ZK60-Ano, but comparatively larger in case of AZ31-Ano. Similarly, the SEM images also indicate a significant number of cracks in AZ31-Ano as compared to ZK60-Ano, which generally influences the mechanical behavior of the coating. The measured stiffness plots indicate the linear behavior with incremental rise with indentation depth as shown in Figs. 4b and 5b. According to equation 1, S is linearly proportional to Er, therefore the

To access the quality of the anodized films developed on AZ31 and ZK60 alloy samples under optimized conditions (at 20 V and 30 min), the quasi-static nanoindentation test was performed to determine the structural integrity of these anodized film. The Stiffness (S), Hardness (H) and Reduced Elastic Modulus (Er) were calculated from the loading and unloading profiles from the nanoindentation test. It is important to note that the values of the mechanical properties of the film could be influenced by anisotropy and surface orientation. The values are also sensitive to the loading-unloading rates and indentation depth. The hardness may be effected by indentation size effect as well [36]. Oliver and Phar method are usually suitable with Berkovich indenter, which continuously measures the indentation load (P), and displacement (h) throughout the loading and unloading cycles of the hysteresis. The stiffness is measured by fitting the polynomial expression to the unloading curve. The x and y components of the slope can be used to calculate the stiffness according to Eq. (1).

S=

dP 2 = Er A dh

(1)

where A is the area of impression from the plastic deformation and Er is reduced elastic modulus obtained from the combination of indenter and film. For a specific value of Er (Eq. (2)), the film modulus, Ef is calculated from Eq. (2)

(1 vf 2 ) 1 (1 vi2) = + Er Ef Ei

(2)

where Ef and vf are the elastic modulus and Poison's ratio of the film, and Ei and vi are the elastic modulus and Poison's ratio of the indenter, respectively. Hardness is calculated from the Eq. (3)

H=

Pmax A

(3)

where Pmax is the peak load at the hysteresis plot. The quasi-static nanoindentation tests, in which the indenter was driven into a specimen to a certain depth and gradually removed from the specimen. The resulting hysteresis curves are known as the forcedisplacement (P-h) plot. The typical force-displacement (P-h) plots of the AZ31-Ano and ZK60-Ano sample for various maximum indentation depths are shown in Figs. 4a and 5a, respectively. The loading regions of the plots are not superimposing each other, which is due to the

(b)

140k

1 m 2 m 3 m ding

105k

a Lo

n di

g

0.84 0.56 0.28

Elastic Modolus Ef (GPa)

70k

1.12

0.00 25

Unloa

35k

20 15 10 5 0

Stiffness ( N/nm)

Force( N)

Hardness (GPa)

(a)

0

0

1

2

3

440 220 0

1

2

3

Indentation Depth ( m)

Displacement ( m) Fig. 4. Load-indenter displacement (P-h) plots taken at various indentation depths of anodized film on AZ31-Ano. (b) Summary of hardness, film elastic modulus and stiffness. 910

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1m 2 m 3 m

140k

Force (N)

105k

70k

0.92

40

Stiffness ( N/nm)

35k

Hardness (GPa)

(b)

Elastic Modolus Ef (GPa)

(a)

0 0

1

2

3

0.69 0.46 0.23

30 20 10 390 260 130 0

1

2

3

Indentation Depth (m)

Displacement (m) Fig. 5. (a) Load-indenter displacement (P-h) plots taken at various indentation depths of anodized film on ZK60-Ano. (b) Summary of hardness, film elastic modulus and stiffness.

uniform material with a constant elastic modulus value of Er constantly changes with indentation depth. The average stiffness values of ZK60Ano is higher in comparison with that of AZ31-Ano. From the plots, it could be seen that at the lower indentation depths, i.e.,1 & 2 µm, the S values for ZK60-Ano are two times higher (118.23 & 180.06 µN/nm) than AZ31-Ano (57.46 & 106.12 µN/nm). These values get closer with indentation depth of 3 µm, i.e.,416.30 µN/nm in ZK60-Ano and 347.21 µN/nm in AZ31-Ano. The initial lower values corresponds to the porosity of the upper film, however, the film achieved compaction with depth. It is worth to consider that the values of H, S and Ef at closer depth to the substrate could be significantly influenced by substrate effect. The mechanical properties of the coatings are crucial for biomaterials. The strong adhered coating are more reliable in long run implantation and osseointegration. The less adhered coating can lead to implant loosening due to the peeling-off effect. High hardness values of the film also influence the degradation mechanism under physiological environments.

Furthermore, the Mg2p high-resolution spectra were deconvoluted and disclose a peak at BE of 51.4 ± 0.1 eV, which is a typical oxide found in magnesium in its oxidized state as shown in Fig. 6a and b. However, the peak at 52.5 ± 0.1 eV designates the presence of Mg (OH)2 whereas the peak at 53.2 ± 0.1 eV indicates the CO32−. Similarly, in the case of ZK60-Ano, in Mg2p, CO32− is identified at a low peak, BE of 533.5 ± 0.2 eV, present due to the chemisorption of the CO32−. On the basis of the above evidence the XPS data indicated the existence of the MgO, Mg(OH)2, and MgCO3 within the anodized coating. Hence on the basis of the XPS plots in this study, it was assumed that the outer surface of the anodized film consists of Mg(OH)2 and MgCO3 while the inner layer is composed of the MgO with other species. L.Wang et al. confirmed the existence of the Mg(OH)2 and MgCO3 in film's outer layer corroded zone and coexistence of the MgO with Mg (OH)2 and MgCO3 species in the inner layer of the AZ31 and AZ91 alloys [40]. Similarly, Song et al. have previously reported that the surface oxide film of AZ21 consists of three layers, an outer layer of Mg (OH)2, the middle layer MgO [41].

3.3. XPS high-resolution spectra

3.4. Electrochemical analysis and corrosion investigation

To further analyze the nature of species present in the anodized film the survey and high-resolution XPS spectra were obtained. The peak positions of all the species are calibrated using the C1s adventitious peak (binding energy of 285.0 eV [38,39]. The C1s binding energy (BE) relative to MgO on the anodized samples i.e. AZ31 and ZK60, were observed to fluctuate in a broad range (between 280.9 eV and 284.6 eV). Fig. 6 shows the deconvolution of all the high-resolution spectra of O1s and Mg 2p elements by applying the Gaussian and Lorentzian function. The O1s spectrum is deconvoluted into three peaks, which correspond to O, OH and CO3 groups in anodized magnesium. The same groups are detected in FTIR as explained in the previous section. Both anodized samples, the O2− peaks are identified at 530.1 ± 0.1 eV, OH− peak is at 531.8 ± 0.1 eV and CO32− at 533.2 ± 0.2 eV. The development of MgCO3 is related to the diffusion of ambient CO2, which lead to the reaction products of carbon in film [40]. The presence of carbonate components could also convince from the FTIR in Fig. 2 and EDS peaks in Fig. 1, as both the anodized samples, indicate CO32− peak. The formation of MgCO3 The reaction is sensitive to the concentration of moisture in the air as well as in coating.

The human physiological environment is composed of corrosive species of chlorides, amino acids, and proteins. These species provide the means for corroding metallic materials and destabilize its oxide passive film. The potentiodynamic polarization is a tool to study the corrosion of the alloys in a simulated environment. To initiate the process of polarization it is necessary to stabilize the surface of each sample before running the tests, therefore the samples were subjected to open circuit potential. The samples were stabilized by immersion in phosphate buffer saline (PBS) for 6 h, that the corrosion species could intrude the porous film to govern purely the electrochemical responses related to surface topography. Prior to stabilizing the sample surfaces, the potentiodynamic scans were originated from negative cathodic potential (−2.0 V vs SCE) (reducing conditions) to positive 0.5 V and the plots obtained are mentioned in Fig. 7. In both sets of samples, the untreated surfaces show higher current densities (Icorr) while lower current densities were observed in anodized samples. The lower current densities could be associated with the partial dissolution of passive films. The corrosion potential (Ecorr) is more negative (active) in the case of untreated surfaces in comparison with anodized samples in both 911

Applied Surface Science 484 (2019) 906–916

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Fig. 6. Deconvoluted XPS peaks of high-resolution spectra, (a) O1s spectra of AZ31-Ano and (b) Mg2p spectra of AZ31-Ano (c) O1s spectra of ZK60-Ano and (d) Mg2p spectra of ZK60-Ano.

cases of AZ31 and ZK60. The quantitative parameters obtained from the plots are mentioned in Table 2. The cathodic regions exhibit the behavior of the reduction processes on the surface of the samples and are controlled by hydrogen evolution reactions as mentioned in Eq. (1) [42]. The slop of the cathodic regions (βc) in untreated samples has higher slopes in comparison with anodized. The AZ31-Unt has βc of − 103.0 mV/decade, while anodized has shown −148.2 mV/decade, which agrees the rate of reaction or hydrogen evolution is lower in AZ31-Ano. The βc of the ZK60-Ano (−115.7 mV/Decade) is slightly lower than the untreated surface (−129.7 mV/Decade), and it indicates the slight progress in hydrogen evolution on the anodized surface. This behavior could explain by understanding the topographic nature, the chemical composition and its influence on the corrosion processes of the coating.

Table 2 Quantitative analysis of potentiodynamic polarization scans.

AZ31-Unt AZ31-Ano ZK60-Unt ZK60-Ano

Ecorr (mV)

icorr (μA/cm2)

βa (mV/dec.)

βc (mV/dec.)

−1.46 −1.27 −1.49 −1.44

4.46 0.39 12.05 0.72

194.9 222.4 97.45 185.9

−103.0 −148.2 −129.7 −115.7

From the SEM micrograph, we have noticed the micro-cracks in the AZ31-Ano sample and the EDS, FTIR, and XPS indicate the presence of the eCO3−2 on the top layer of the anodized layer. The rigorous reduction starts soon after the immersion of anodized samples, as the PBS intrudes into these micro-cracks and channels. Initially, the pH reduces

Fig. 7. Potentiodynamic plots of magnesium alloys before and after anodization. 912

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(b)

(a)

105 2000 1500

30k 1000

Log Norm |Z| (ohm.cm2)

AZ31-Unt AZ31-Ano Simulated

500

20k

0 -500 -500

0

500 1000 1500 2000

10k

20

AZ31-Unt AZ31-Ano Simulated

10 0

104

-10 -20 103

-30 -40 -50

2

10

Phase Angle (Degree)

- Norm Zimag (ohm.cm2)

40k

-60 -70

0 0

10k

20k

30k

101 10-2

40k

Norm Zreal (ohm.cm2)

10-1

100

101

102

103

104

105

Log F(Hz)

Fig. 8. EIS overlapped spectra of AZ31-Unt and AZ31-Ano samples. (a) Nyquist plots (b) Bode plots.

at the pores due to the reduction of MgCO3 into carbonic acid (HCO3). The cracks are in higher energy states and the rate of reactions is higher, therefore the Mg(OH)2 is produced which starts to precipitate. The formation of Mg(OH)2 fill up the cracks and channels soon started inhibition of the corrosive species to reach beyond this new barrier. The typical reactions take place are given in Eqs. (4) and (5).

2H2 O + 2e

H2 + 2OH

Mg2 + + 2OH

Mg(OH) 2

from PBS, and have no further activity within the film. Similarly, the MgCO3 was only present in the top layer and which have the susceptibility to react with OH− ions and released to the PBS as HCO3 during surface stabilization procedure. To further understand the corrosion of the coatings based on the response of electrode kinetics, EIS was conducted. The electrode kinetics at the interface give a specific response when an AC signal of a small amplitude (10mV) is applied. The impedance response of the AZ31 samples in PBS is given in the Nyquist plot in Fig. 8a. The plot shows one capacitive loop at the higher to the intermediate frequency range and another in the lower frequency range in AZ31-Unt. However, the anodized samples show, intermediate and low-frequency loops with inductive characteristics in the low-frequency regime. Similarly, in the case of ZK60, the anodized sample have larger semi-circle in comparison with the untreated sample as shown in Fig. 9a. No inductive characteristics have shown by the AZ31Unt sample. To elaborate the explanation in order to understand the real-time corrosion behavior the bode plots impedance and phase angle were analyzed. The Figs. 8b and Fig. 9b show the impedance bode plot of the AZ31 and ZK60 samples respectively. In AZ31, the low-frequency region of the impedance modulus could be used as a gauge of the coating efficiency in terms of resistance; therefore, increase in the impedance from 5.6 × 103 Ω-cm2 to 2.966 × 104 Ω-cm2 indicates the stability and protection efficiency of the anodized layer. The increase in the impedance at lower frequency region is attributed to the formation of the corrosion products that accumulate on the surface. The impedance is high in case of the anodized sample at each frequency function than AZ31-Unt until the end of the test. Similarly, in the case of ZK60, the impedance of the anodized sample at the higher frequency region shows variations, which is an indication of the formation of the corrosion products on the surface. Both AZ31-Unt and ZK60-Unt presented two-time constants at high to intermediate and low-frequency regions while anodized samples show three-time constants at broadening frequency range with increasing phase angles. According to song et al., the dissolved Mg+ ions are accountable for the intermediate frequency capacitive and lowfrequency inductive loop, whereas capacitive loop in high-frequency range attributed to the charge transfer resistance and capacitance [43]. To evaluate the quantitative parameters of the processes, the plots were fitted with equivalent electrical circuits (EEC) to estimate the electrochemical properties of the corroding system. The EEC are presented in Fig. 10 while the solid line in Figs. 8 and Fig. 9 show the simulated plots. The impedance parameters derived are given in Table 3. In the EEC, each component has an implication; Rs indicates solution resistance between the reference electrode and the

(4) (5)

This behavior could recognize from the cathodic region in AZ31. Initially, the slope in the highlighted cathodic region is higher and sudden change after a decade millivolts the slope become linear as shown in Fig. 7. The initial change in slope might be attributed to the ingress of the corrosion species and formation of Mg(OH)2 and the linear region could be stability attributed to the channels after the formation of the barrier layer. However, in ZK60 the cathodic regions have shown linear behavior since the start of the cathodic polarization, which ascribes the compaction of the anodized layer. From the plots, it is observed that the AZ31-Unt have Icorr (4.46 μA/ cm2) shifted to lower current densities after electrochemical anodization (394.8 e−3 μA/cm2). Similarly, in the case of ZK60, the shift is more significant and the Icorr values shifted from 12.05 μA/cm2 to 714.8 e−3 μA/cm2. The lower current density observed in anodized samples and having relatively less negative potentials, which could be associated with the stability of the film with the limited dissolution compare to the higher current densities of untreated samples. However, in the anodic regions, the anodized sample depicts the breakdown potential of the film as shown in Fig. 8. The possible reactions take place during the breakdown initiation of the film could provide insight into the corrosion process are shown in Eq. (6), (7) and (8).

Mg

Mg2 + + 2e (Anodic reaction)

2H2 O + 2e

H2 + 2OH (Cathodic reaction)

Mg2 + + 2OH

Mg(OH) 2 (Product formation)

(6) (7) (8)

The existence of chloride ions in the PBS could further react with Mg (OH)2 and form a more soluble MgCl2 so that Mg(OH)2 protective layer is dissolved given in Eq. (9).

Mg(OH) 2 + 2Cl

MgCl 2 + 2OH

(9)

The formation of the MgCl2 has not been detected, however, it was considered, as it is the main constituent of the PBS. The byproducts in the above chemical equation are obvious from the XPS data. However, it is noticed that PO4 and F came from the physisorption of salt crystals 913

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(a)

(b) 105 600

400

15k

Log Norm |Z| (ohm.cm2)

ZK60-Unt ZK60-Ano Simulated

200

10k

0 0

200

400

600

5k

40

ZK60-Unt ZK60-Ano Simulated

104

30 20 10 0 -10

103

-20 -30 -40

2

10

Phase Angle (Degree)

- Norm Zimag (ohm.cm2)

20k

-50

0

-60 0

5k

10k

15k

101

20k

10-2

10-1

100

Norm Zreal (ohm.cm2)

101

102

103

104

-70

105

Log F(Hz)

Fig. 9. EIS spectra of ZK60-Unt and Anodized samples. (a) Nyquist plots (b) bode plots and.

in Mg dissolution. Rf and Lf are also pseudo-resistance and inductance, causing by the adsorption of the intermediate species on the electrolyte/substrate double layer due to the breakdown of the substrate surface film [43,44]. The capacitive loop at the high-frequency region is related to the resistance of the double layer of anodized film, the capacitive loop at the medium frequency is related to the resistance of charge transfer at double layer capacitance at the bottom of the porous coating. The small and low-frequency loop signifies mass transport relaxation due to the diffusion of Mg+ ions through the corrosion layer, and adsorption if the Mg+ intermediates. It is observed that the film resistance is slightly increased in the anodized samples compared to that of untreated samples, which is an indication of better barrier properties of the anodized passive film. Similarly, the charge transfer resistance Rct is significantly improved, due to the presence of MgO, Mg(OH)2 with traces of the MgCO3 confirmed by the FTIR and XPS. The CPE parameter of the high-frequency time constant (Yf) shows a slight increase in anodized samples in comparison with untreated surfaces. In case of AZ31, CPE values of the film (Yf) jumped from 0.38 μS·sn/cm2 to 7.82 μS·sn/cm2 while in case of ZK60 0.414 μS·sn/ cm2 to 3.29 μS·sn/cm2. Nevertheless, the resistance of the film Rf or pore resistance increased in anodized surfaces and the CPE values are slightly increased. This might be affiliated to the formation of the active sites where uniform growth and localized dissolution imposed by the synergistic effects of Mg2+ and OH− combine and form Mg(OH)2 to generate resistive barrier within the pores by same chemical reaction mentioned in Eq. (6). The formation Mg(OH)2 could correspond to the concentration of Cl− ions, OH− and PO43− ions of the PBS, increases within the pores, consequently damage to the coating occurs. This behavior can correspond to the related components of L and RL. The medium to low-frequency time constant was fitted for the purpose of the double layer non-ideal capacitance and resistance at the double layer. In AZ31 the CPE of double layer Ydl is lowered in anodized samples (3.08 μS·sn/cm2) and the Rct is higher (28.4 KΩ-cm2) indicates effective protection of the substrate from corrosion and the defective

Fig. 10. Electrical equivalent circuit model used to simulate the impedance spectra (a) AZ31-Unt (b) ZK60-Unt and (c) AZ31-Ano and ZK60-Ano.

working electrode, Yf is the non-ideal capacitive behavior of the solution/oxide film interface. Rf signifies the resistance of the anodized film whereas the Yf and Rf are introduced in untreated surfaces because of the formation of the Mg(OH)2 during surface stabilization in OCP. Ydl is the capacitance of the double layer, Rct signifies the charge transfer resistance at the interface of film/substrate within the pores. The Ydiff and Rdiff indicate the pseudo capacitance and resistance caused by Mg+ Table 3 Impedance parameters obtained by fitting EEC models for AZ31 and ZK60.

AZ31-Unt AZ31-Ano ZK60-Unt ZK60-Ano

Rs (Ω-cm2)

Yf (μS·s /cm )

n1

25.77 126.0 20.83 8.74

0.38 7.82 0.41 3.29

0.74 0.77 0.72 0.76

n

2

Rf (Ω-cm2)

Ydl (μS·s /cm )

n2

86 665.8 64.17 416.20

6.05 2.90 0.66 9.49

0.85 0.78 0.73 0.71

n

2

Rct (Ω-cm2) 1164 28,300 480.7 1911

914

Ydiff n

2

(μS·s /cm )

n3

– 252.7 – 52.24

– 0.02 – 0.16

Rdiff (Ω-cm2)

L

RL

Goodness of fit

– 5275 – 32,130

– 0.04 2770 13,700

– 11,740 1080 4510

2.2E-03 902E-03 353E-06 575E-06

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Fig. 11. Cell morphology and adhesion of the anodized surface of: AZ31-Ano and B: ZK60-Ano after incubated for 48 h.

sites at the double layer of the AZ31 are shielded by the anodized layer, however the case is contrasting in ZK60. The charge relaxation is higher in comparison with that of the anodized surface, which indicates the porosity of the inner layer and unequal distribution of the active sites, however, the charge transfer resistance is significantly increased (1.911 KΩ-cm2) relative to ZK60-Unt (480.7 Ω-cm2), which shows the stability of the oxide film. At the small capacitive loop, the mass transfer and diffusion of Mg+ related to the adsorption ions at the double layer. In AZ31-Ano Ydiff is higher, indicates the reactivity and compromises the Rdiff. In the case of ZK60, the Ydiff is smaller and Rdiff is higher and shows the comparatively better stability of the porous surface. To summarize, the above results indicate an effective barrier properties to serve as a better biomedical biodegradable coating for magnesium alloys. The anodized layer provides sufficient protection to enhance dissolution time within a physiological environment for the alloys. The new coating protects the active corrosion sites by synergistic effect and thus control simultaneous degradation and expected to enhance bio-functionality.

5. Conclusions Magnesium alloys were electrochemically anodized in the alkaline electrolyte by optimizing voltage and time (20 V and 30 min). The surface chemistry analysis showed that the anodized film consists of MgO, Mg(OH)2 and traces of MgCO3 in both AZ31 and ZK60. The surface roughness analysis showed that the ZK60 anodized (473.54 ± 51.61 nm) film has a higher roughness when compared with that of anodized AZ31(112.11 ± 11.31 nm). The ZK60-Ano has an average hardness of ~0.49 GPa, greater than that of AZ31-ANO (~0.35 GPa). Film modulus (Ef)increases from 7.8 GPa at 1 µm and reaches ~20 GPa at 3 µm. However, in case of ZK60-Ano, the Ef values remain about ~20.04 GPa for all indentation depths. Similarly, stiffness values are also higher in ZK60-Ano as compared to AZ31-Ano. The investigation of the corrosion mechanism was conducted and comparisons were made with respective untreated surfaces. The anodized coating effectively constrains the corrosion of Mg alloys, by hindering most of the substrate surface area by reducing the ingress of the corrosive electrolyte and passivating the substrate defective/active points. The potentiodynamic polarization scans display the relative low Icorr values in anodized samples are shifted from 4.46 μA/cm2 to 394.8 e−3 μA/cm2 in AZ31 and 12.05 μA/cm2 to 714.8 e−3 μA/cm2 in ZK60 after anodization. The results were guaranteed by the electrochemical impedance spectroscopy by indicating that the porous anodized layer controlled the dissolution rate by enhanced charge transfer resistance (Rct) from 1.164 KΩ-cm2 to 28.3 KΩ-cm2 in AZ31 while in ZK60 this value shifted from 0.481 KΩ-cm2 to 1.911 KΩ-cm2. The cell proliferated showed stiller morphological structure which indicates the enhanced biocompatibility of the new film.

4. Cell surface interaction Metal and alloys have potential toxicity, however; it depends on the oxidation state of the material, concentration and absorption capability of the ions from the respective material. Therefore, it is important to investigate the metallic toxicity in the close proximity of the material, which directly influences the cell viability, morphology, and osseointegration [45]. Moreover, cell adhesion and osseointegration could be influenced by the implant surface topography and are the initial standards to define the quality of osseointegration in a cascade of cell implant interactions. In this study, the potential toxicity of the anodized surfaces was studied by simply proliferating MC3T3 preosteoblast cells on the surfaces. After the 24 h of incubation, the cells showed normal proliferation in both AZ31-Ano and ZK60-Ano samples. The cell morphological structure could be clearly seen in Fig. 11. From the FTIR and XPS data, all the chemical components of the anodized layer comprised of biocompatible species. The EIS and potentiodynamic data show the significant decrease in dissolution rate and higher charge transfer resistance, which indicates the controlled release of the ionic species to the electrolyte. This could be directly related to the less metallic ions interaction with the cells. The AZ31 and ZK60 anodized surface show a biocompatible nature of its chemical and electrochemical nature. Fig. 11 shows a magnified image of the cells structure, the cells show a normal morphological structure with a well-spread network of filopodia and lamellipodia, which is an indication of the physical adhesion of the cells to the implant, and ultimately better osseointegration.

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